Footballer Marcus Rashford is calling on health professionals to do more to persuade families in need not to miss out on government food vouchers.
In an open letter in the British Medical Journal (BMJ), he says more than 40% of those entitled to claim have not signed up.
While thousands have benefitted, he is concerned numbers are levelling off due to a lack of awareness.
Ministers say they are spending more than £400m to help low income families.
Those who qualify receive vouchers to buy basics like milk and fruit or coupons for vitamins.
The pandemic has put a renewed focus on child poverty but the England and Manchester United striker says more needs to be done to end any "silly" stigma and to persuade people to register for support.
"We need you"
Writing in the BMJ, Rashford says his Child Food Poverty Taskforce - set up to campaign for vulnerable children to receive free meals during the school holidays - has been doing all it can to raise awareness about the voucher scheme.
He writes: "While we have seen 57,000 more parents benefit from the scheme as a result, I'm concerned we are plateauing.
"More than 40% of those eligible for the vouchers are still not registered for the scheme, and I'm confident that the majority of these parents can be found in communities just like mine, where I grew up - no internet, no high street, no word of mouth."
And he says health professionals need to do more to ensure all those who are entitled to apply, know about it "especially given the planned digitisation of the scheme this autumn, which will disproportionately disadvantage those without easy access to the internet".
He wants staff to use an online eligibility calculator and "consider collaborating with us on communicating and educating people about the scheme when possible".
He adds: "In conclusion, and not to beat around the bush, we need you."
Last summer, the government was forced in to a u-turn, after a high-profile campaign by Rashford for free school meal vouchers to be available for six weeks during the holidays.
The government had previously insisted that free school meals would only be provided during term time, but in June 2020, the prime minister said they had to understand the pressure families were under and a Covid summer food fund would help.
In November, ministers provided more than £400m for a winter grant scheme, run by councils, to provide support with food and bills.
'Doing government's work'
Labour's shadow child poverty secretary, Wes Streeting, said the measures did not go far enough and "more and more children are living in poverty under the Tories".
He said: "With no government strategy for lifting children out of poverty, once again we see Marcus Rashford having to do the government's work for them.
"If Boris Johnson is out of ideas for what to do, he should implement Labour's Children's Recovery Plan to provide immediate support for all children to catch up on lost learning following the pandemic."